1. Field of the Invention
This invention relates to a method and an apparatus for distinguishing a latent fingerprint from a fingerprint of a biomass. Particularly, the method can prevent a fingerprint recognition device from mis-recognizing a latent fingerprint as a fingerprint of a biomass due to a fingerprint residual on the imaging surface of an optical fingerprint input apparatus.
2. Description of the Prior Art
A fingerprint recognition device can be utilized on a wide scale as a device for comparing and recognizing fingerprints between an inputted fingerprint and a pre-registered fingerprint. A fingerprint recognition device is used as a locking mechanism of a door or a safe, access control to a gate, attendance control of employees, access control to a computer, or various other controls. The fingerprint input apparatuses for receiving fingerprints to perform fingerprint recognition are mainly classified into two types: an optical type and a non-optical type. The fingerprint recognition device employing an optical fingerprint input apparatus is a device that initially illuminates a fingerprint laid on a prism, interprets the fingerprint image reflected according to the shapes of valleys or ridges of the fingerprint and formed on an image sensor, and then compares the interpreted image with a pre-stored fingerprint.
The optical fingerprint input apparatuses are mainly classified into an absorption type and a scattering type.
FIG. 1 is a schematic diagram illustrating an operational principle of a fingerprint input apparatus of an absorption type, which comprises a backlight 112, a triangular prism 110, a lens 114, an image sensor 116, and an image processor 125. The backlight 112 uses a plurality of LED aligned. The triangular prism 110 is a prism of a right triangular shape that generates a total reflection inside an imaging surface when no fingerprint is inputted. The image sensor 116 is an element outputting electric signals corresponding to an amount of inputted light, such as a CCD or a CMOS sensor, well known to those skilled in the art. The inclined surface of the triangular prism 110 is an imaging surface, while an internal plane of the imaging surface 118 is a total reflection surface causing the total reflection.
Under no input of a fingerprint to the imaging surface 118, the light originating from the backlight 112 is totally reflected from inside the imaging surface of the triangular prism 110, and is incident on the image sensor 116 through the lens 114. If a finger is laid on the imaging surface, the light illuminated onto the valleys of the fingerprint is totally reflected from the internal surface of the imaging surface 118 and reaches the image sensor 116 because the valleys of the fingerprint are not in contact with the imaging surface. By contrast, the light illuminated onto the ridges of the fingerprint is not totally reflected from the internal surface of the imaging surface 118 but rather, only a part of the reflection reaches the image sensor 116.
Accordingly, the amounts of light incident on the image sensor 116 differ between the valleys and the ridges, and as a consequence, the image sensor 116 outputs electric signals of different levels depending on a pattern of a fingerprint. The image processor 125 formulates the output values of the image sensor 116 into digital signals so as to recognize a fingerprint pattern.
FIGS. 2A and 2B are schematic diagrams illustrating an operational principle of a fingerprint input apparatus of a scattering type.
The fingerprint input apparatus in FIG. 2A comprises a backlight 212, a prism 210, a lens 214, and an image sensor 216 with a similar construction to the one in FIG. 1. However, the prism 210 is of a ladder shape rather than a triangular shape. Unlike the absorption type shown in FIG. 1, the light is incident on the imaging surface 218 of the prism 210 from the backlight 211 at an angle far smaller than the right angle or a critical angle. Therefore, the light illuminated onto the valleys of the fingerprint not in contact with the imaging surface 218 penetrates the imaging surface 218 and does not reach the image sensor 216. Meanwhile, the light illuminated onto the ridges of the fingerprint is scattered by the ridges. The scattered light is incident on the lens 214 and is sensed by the image sensor 216.
FIG. 2B is a schematic diagram illustrating an operational principle of the fingerprint input apparatus of another scattering type. As in the case of FIG. 2A, the light illuminated onto the valleys of a fingerprint penetrates the imaging surface 318 and does not reach the image sensor 316. The light illuminated onto the ridges of the fingerprint is scattered by the same principle. However, the difference lies in using a prism of an isosceles triangular shape and changing the position of the backlight 312.
In case of the fingerprint input apparatus of an absorption type, the light is absorbed at the ridges of a fingerprint. Therefore, the image of the fingerprint appearing on the image sensor is dark at the ridges and bright at the valleys. In case of the fingerprint input apparatus of a scattering type, however, the light is scattered at the ridges of a fingerprint. Therefore, the image of the fingerprint appearing on the image sensor is a bright image at the ridges and dark at the valleys, thereby reflecting a comprehensively contrary image to that of the fingerprint input apparatus of an absorption type. To facilitate processing the fingerprint image as well as to avoid an inversion of bright and dark images of a fingerprint appearing on a monitor of a computer depending on the input methods, an inversed image is displayed on the monitor of a computer in the case of the fingerprint input apparatus of a scattering type. To be specific, although the actual fingerprint image appearing on the image sensor is bright at the ridges and dark at the valleys of the fingerprint, the gray level in the course of processing the fingerprint image his a low value at the ridges and a high value at the valleys as in the case of the fingerprint input apparatus of an absorption type.
In case of the optical fingerprint input apparatus, however, sebum or a contaminated material leaves a latent fingerprint on the fingerprint recognition apparatus due to contact with a person's finger. If a light is incident on the imaging surface from an external light, rather than from a backlight, at a particular angle, the image sensor is apt to sense a latent fingerprint. Thus, if the image sensor senses any latent fingerprint, the fingerprint recognition apparatus mis-recognizes the latent fingerprint as a fingerprint of a biomass. This causes a problem in that an unauthorized user may be authenticated for access by using the latent fingerprint left on the fingerprint recognition apparatus instead of inputting his or her own fingerprint.
FIG. 3A shows an image of a normal fingerprint of a biomass, and FIG. 3B shows a clear image of a latent fingerprint, which is quite similar to the one in FIG. 3A. FIG. 3C shows a vague image of a latent fingerprint.
To solve the problem of mis-recognizing a latent fingerprint, the conventional art uses a method of storing the most recently inputted fingerprint of a person, comparing the stored fingerprint with the currently inputted fingerprint, and distinguishes the newly inputted fingerprint from the fingerprint of a biomass as a latent fingerprint if the two fingerprints are quite similar (i.e., when the positions of a particular point of the two fingerprints coincide with each other or when comprehensive patterns of the two fingerprints overlap with each other).
However, this method still poses a problem in that a pattern of the latent fingerprint read by the image sensor is variable due to a change of the external light, and due to other factors, and the stored pattern may be distinguished as different from the latent fingerprint. The image sensor therefore may fail to discriminate a latent fingerprint from a biomass fingerprint accurately.